A “Tail“ of Origin: How The Rattlesnake Got Its Rattle

By Michael DeLea

            If you’re anything like me, you have spent many a night lying wide-awake in bed, racking your brain for the answer to the following question: How in the name of evolution did the rattlesnake get its rattle? Well, you’re in luck because that is exactly the topic I intend to tackle with this blog post!

Crotalus atrox demonstrating typical defensive posture

            Rattlesnakes are unique among vertebrates in possessing the unusual structure that we commonly refer to as a rattle. This innovative structure is the result of numerous anatomical, physiological, and behavioral specializations and has been the object of interest and speculation for decades. Additionally, it occurs in less than 30 of the approximately 2,700 known species of snakes in the world. The structure and complexity of the rattle suggests that it arose only once in a shared common ancestor as opposed to evolving independently in each of the different species of New World pitvipers.

            Rattlesnake neonates are born with what is referred to as a “button” at the tip of the tail that is bulbous with two annular constrictions that prevent it from being shed. This is the site of actual rattle segment formation and consists of the bony, club-like style that is attached to the terminal region of the caudal vertebrae, the fibrous connective tissue that forms the rattle segment mold and the epithelial tissues. Every time a rattlesnake sheds, a segment is added to the base of the rattle. A juvenile snake may add three to four segments each year where an adult may only add one segment annually. This notion, and the fact that rattles may break if they become too large, disprove the myth that you can determine the age of a rattlesnake by counting the number of segments.

Rattles with varying numbers of segments

            Rattling in these snakes is accomplished by shaking the rattle structure at very high frequencies with the use of specialized muscles that are attached to the modified caudal vertebrae. Three pairs of muscles that contract out of phase causes the segments to rub against one another, producing the unmistakable sound. What is interesting about the “tailshaker” muscle group is that it contains no apparent unique structures. Instead, the proportions of the structures that are common to all skeletal muscles are suited to minimize activation, contraction, and relaxation times and to maximize ATP production.

            Now that you are familiar with the mechanism behind the rattling behavior from which the name is derived, let’s talk about the potential adaptive significance of the structure in these organisms. While the rattle itself may be exclusive to the New World pitvipers, tail vibration is not. In fact, defensive tail vibration is quite common in snakes, including many non-venomous species. So why, then, did the common ancestor evolve this unique morphological structure? There have been many hypotheses attempting to explain the origin of the rattle. These potential explanations range from outlandish (i.e. snakes use their rattle to “charm” and immobilize prey) to plausible. I will discuss the three main hypotheses that are currently the most appealing amongst the herpetological community.

            The first, and most popular hypothesis is that rattles evolved to serve as an aposematic warning of the snake’s venomous nature. It is thought that snakes evolved in the montane regions of Mexico, where they would have encountered small predators such as the White-nosed Coati (Nasua narica) and the Ringtail (Bassaricus astutus). It is therefore reasonable to think that the rattle was used to warn these predators of the danger that would follow if they were to engage the rattlesnake.

            Another hypothesis is that the rattle could be used to attract prey into the strike range of the rattlesnake, as demonstrated in the feeding strategy known as caudal luring. This behavior has been documented in many species of extant snakes, including rattlesnakes. In this sense, the rattle could have evolved to enhance the attractiveness of the “lure,” perhaps mimicking the insect prey of frogs or lizards.

Caudal luring + Camouflage = Deadly Combination

            The final hypothesis is that the tail vibration behavior (in venomous and non-venomous snakes alike) is used to bring attention to the snake’s tail instead of its more vulnerable head. If this were the case, then having a rattle would only increase the effectiveness of the distraction when confronted with a predator. This hypothesis may be the most difficult to believe as a result of the recent discovery of at least two populations of rattle-less rattlesnakes. The Santa Catalina Island Rattlesnake (Crotalus catalinensis) and the San Lorenzo Island Rattlesnake (Crotalus ruber lorenzoensis) populations are characterized by a high frequency of individuals lacking a noise-producing rattle. A possible explanation for this occurrence is the unusual arboreal habits of these snakes. It is not difficult to imagine how having a noisy rattle might be a hindrance while hunting prey in trees.

            There has not been conclusive evidence supporting any of these theories so the topic continues to be hotly debated. New evidence, such as the loss of rattles in certain species and subspecies, will be helpful in the attempts of biologists to piece together the evolution of the rattle and the phylogenetic relationships can be determined as a result.

References:

Meik, J.M., and A. Pires-daSilva. 2009. Evolutionary morphology of the rattlesnake style. BMC Evolutionary Biology 9: 35-43.

Moon, B.R. 2001. Muscle physiology and the evolution of the rattling system in rattlesnakes. Journal of Herpetology 35: 497-500.

Rowe, M.P., T.M. Farrell, and P.G. May. 2002. Rattle loss in pygmy rattlesnakes (Sistrurus milliarius): causes, consequences, and implications for rattle function and evolution. Biology of the Vipers: 385-404

Schaeffer, P.J., K.E. Conley, and S.L. Lindstedt. 1996. Structural correlates of speed and endurance in skeletal muscle: the rattlesnake tailshaker muscle. The Journal of Experimental Biology 199: 351-358

Photo Credit:

http://www.theradzoo.com/wp-content/uploads/2012/04/800px-Crotalus_atrox_USFWS.jpg

http://sdsnake.com/Snake/rattle_2X.jpg

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